The investigators propose to develop a sophisticated, yet accessible mathematical model that closely mimics percutaneous penetration, tissue concentrations and clearance in human skin in vivo. This development will significantly advance the mechanistic understanding of allergic and irritant dermatitis and the mechanics of dermal exposure assessment. This objective will be accomplished through the construction and experimental verification of physiological and physical properties-based models for percutaneous absorption incorporating features not found in the mathematical models presently used to estimate dermal exposure. Successful completion of the specific aims of the proposal will lead to the following: (1) an experimentally verified, microscopic model of transport in human stratum corneum and skin appendages (hair follicles, sweat glands); (2) a dermal vascular model that allows accurate predictions of permeant concentrations and clearance in the viable skin layers; (3) a transient diffusion model for calculating dermal absorption subsequent to low-to-moderate exposures to potentially volatile compounds; (4) a skin hydration model for linking the above with steady-state permeability models presently in use; and (5) an easy-to-use spreadsheet for dermal exposure calculations incorporating the most important features from the investigation. The research methods include: Sophisticated numerical techniques for modeling transport in heterogeneous structures and advanced graphical representation of these results. Detailed examination of stratum corneum ultra structure and microtransport properties using laser scanning confocal microscopy, fluorescence photobleaching,and other light microscopic techniques. Determination of diffusivities and partition coefficients of selected permeants in stratum corneum, epidermis, dermis, and hypodermis and the role of protein binding on these values.